This invention relates to pneumatic tires reinforced by aramid cables having specific twist structures.
Twisting a yarn or cable generally increases its overall stretchability and reduces modulus. It is recognized in the tire art that the twist structure of reinforcing cables is dependent upon the material in the cable and its intended use. For example: U.S. Pat. Nos. 2,053,123, 2,132,492, 2,235,082, 2,235,867 and 3,672,423 teach twist structures for rayon cables; U.S. Pat. Nos. 2,273,200, 3,6724423 and 3,851,692 teach twist structures for nylon cables; U.S. Pat. Nos. 3,672,423 and 3,851,692 teach twist structures for polyester cables; U.S. Pat. Nos. 3,029,590, 3,395,529 and 3,554,260 teach twist structures for glass cables; U.S. Pat. Nos. 4,349,063, 4,408,444 and 4,606,392 teach twist structures for steel cables; U.S. Pat. Nos. 3,851,692 and 3,851,693 and published U.K. Patent Applications Nos. 2,042,429A and 2,l63,709A teach twist structures for aramid cables; and U.S. Pat. Nos. 3,977,172 and 4,155,394 teach twist structures for yarns of aramid merged with yarns of other materials in the same cable.
Tire reinforcement fabrics have been made with cables of somewhat extensible or stretchable nylon and/or polyester yarns which can repeatedly be stretched for relatively short extensions and returned to their original length with only minimal permanent deformation. However, for many applications, it is highly desirable that tire reinforcement cables have a substantially greater modulus and breaking strength than that which is ordinarily obtainable from nylon and polyester yarns. In this regard, a suitable material which can provide such high modulus and breaking strength is a yarn of essentially inextensible or non-stretchable aromatic polyamide, generally known as aramid. As used herein, and in the appended claims, "aramid" and "aromatic polyamide" are both understood to mean a manufactured fiber in which the fiber-forming substance is generally recognized as a long chain synthetic aromatic polyamide in which at least 85% of the amide linkages are attached directly to two aromatic rings. Representative of an aramid or aromatic polyamide is a poly(p-phenyleneterphthalamide).
However, such aramid yarns are typically deficient in their resistance to flex and cyclic fatigue, largely due to their characteristically high modulus. In the past, pneumatic tires having more than one carcass ply of aramid fabric have been generally recognized as being deficient in compression fatigue performance. There are disclosed herein pneumatic tires reinforced with aramid cables of particular twist structures, which tires have performed satisfactorily in durability tests.